The DOM MouseEvent represents events that occur due to the user interacting with a pointing device (such as a mouse). It's represented by the nsINSDOMMouseEvent
interface, which extends the nsIDOMMouseEvent
interface.

The button number that was pressed when the mouse event was fired: Left button=0, middle button=1 (if present), right button=2. For mice configured for left handed use in which the button actions are reversed the values are instead read from right to left. Read only.